The present invention relates to a control apparatus for a vehicle that is used in a vehicle capable of traveling at a low speed by causing a drive motor to output creep torque even when accelerator operation is not being performed by the driver.
As described in Japanese Laid-Open Patent Publication No. 11-285108, for example, some vehicles including a drive motor as a drive source, such as hybrid vehicles and electric vehicles, are capable of traveling at a low speed by causing the drive motor to output creep torque even when accelerator operation is not being performed by the driver.
In the vehicle described in the aforementioned document, if brake operation is performed by the driver under a condition in which the drive motor is outputting the creep torque, the creep torque decreases as the amount of the brake operation increases. This reduces the power consumed by the drive motor compared to a case in which the creep torque at the time the brake operation is being performed by the driver is maintained at the creep torque (a predetermined creep torque) immediately before the brake operation is started.
In a case in which creep torque decreases in response to brake operation by the driver as in the case of Japanese Laid-Open Patent Publication No. 11-285108, vibration can occur in the torque transmission system for transmitting output torque from the drive motor to the driven wheels.
Referring to the diagrams of FIGS. 12A and 12B representing a torque transmission system, an example of a factor causing vibration through creep torque decrease will now be described. The torque transmission system illustrated in FIGS. 12A and 12B includes a gear Gm, which is connected to a drive motor, and a gear Gt, which is connected to the driven wheels. The gears Gm, Gt are meshed with each other.
When the driven wheels are rotated by creep torque output by the drive motor, the gear Gm rotates in a predetermined direction X as illustrated in FIG. 12A. This causes a tooth surface Gm1 of a tooth located forward in direction X in the gear Gm, which is connected to the drive motor, to contact a tooth surface Gt2 of a tooth located rearward in direction X in the gear Gt, which is connected to the driven wheels. Creep torque is thus transmitted to the Gear Gt through the gear Gm. In this manner, the creep torque is transmitted to the driven wheels through the torque transmission system, thus rotating the driven wheels to cause the vehicle to travel. This state of the torque transmission system, in which the drive motor drives the driven wheels, is referred to as a drive state.
As the creep torque output by the drive motor is gradually decreased in the drive state, the creep torque transmitted from the drive motor to the gear Gm and the torque transmitted from the driven wheels to the gear Gt become substantially equilibrated with each other. This causes the tooth surface Gm1 of the gear Gm to separate from the tooth surface Gt2 of the gear Gt, thus blocking transmission of the creep torque output by the drive motor to the driven wheels.
As the creep torque output by the drive motor is decreased further, the torque transmitted from the driven wheels to the gear Gt exceeds the creep torque transmitted from the drive motor to the gear Gm. Then, as illustrated in FIG. 12B, a tooth surface Gt1 of a tooth located forward in direction X in the gear Gt, which is connected to the driven wheels, is caused to contact a tooth surface Gm2 of a tooth located rearward in direction X in the gear Gm, which is connected to the drive motor. Such contact between the tooth surface Gt1 and the tooth surface Gm2 causes vibration in the torque transmission system. This state of the torque transmission system, in which the driven wheels drive the drive motor, is referred to as a driven state.
As a method for restraining vibration caused by the above-described changes in contacting tooth surfaces of the gears Gm, Gt, which is, in other words, backlash, of the torque transmission system, creep torque may be maintained at the aforementioned predetermined value even if brake operation is performed by the driver under a condition in which the creep torque is being output by the drive motor. Then, as the amount of brake operation increases, brake torque applied to the driven wheels by the brake device is increased. This method maintains the torque transmission system in the drive state and restrains transition of the torque transmission system to the driven state. Vibration due to the backlash is thus unlikely to be generated in this torque transmission system. However, if the method is employed, the creep torque is maintained without being decreased even if brake operation is performed by the driver. This increases the power consumed by the drive motor.